1. Field of the Invention
The present invention relates to data transmission systems and, more particularly, to transceiver initialization in data transmission systems.
2. Description of the Related Art
Data transmission systems which use multicarrier modulation have been receiving a lot of attention due to the high data transmission rates they offer. There are many different multicarrier modulation techniques that can be used in such systems. One popular multicarrier modulation technique is known as Discrete Multi Tone (DMT) modulation. Other multicarrier modulation techniques include Discrete Wavelet Multi Tone (DWMT) modulation and Orthogonal Frequency Division Multicarrier (OFDM) modulation.
One standard for high-speed data transmissions over twisted-pair phone lines that has developed is known as Asymmetric Digital Subscriber Lines (ADSL). The standard for ADSL is commonly known as T1E1 ADSL Standard of American National Standard Institute (ANSI). Another standard for high-speed data transmissions over twisted-pair phone lines that is presently proposed is known as Very High Speed Digital Subscriber Lines (VDSL). FIGS. 1A and 1B are basic exemplary diagrams of a transmitter and a receiver of a multicarrier modulation transmission system suitable for use with ADSL and VDSL.
FIG. 1A is a block diagram of a conventional transmitter 100 for a multicarrier modulation transmission system. The transmitter 100 receives data signals to be transmitted at a buffer 102. The data signals are then supplied from the buffer 102 to a forward error correction (FEC) unit 104. The FEC unit 104 compensates for errors that are due to crosstalk noise, impulse noise, channel distortion, etc. The signals output by the FEC unit 104 are supplied to a data symbol encoder 106. The data symbol encoder 106 operates to encode the signals for a plurality of frequency tones associated with the multicarrier modulation. In assigning the data, or bits of the data, to each of the frequency tones, the data symbol encoder 106 utilizes data stored in a transmit bit allocation table 108 and a transmit energy allocation table 110. The transmit bit allocation table 108 includes an integer value for each of the carriers (frequency tones) of the multicarrier modulation. The integer value indicates the number of bits of data that are to be allocated to the particular frequency tone. The value stored in the transmit energy allocation table 110 is used to effectively provide fractional number of bits of resolution via different allocation of energy levels to the frequency tones of the multicarrier modulation. In any case, after the data symbol encoder 106 has encoded the data onto each of the frequency tones, an Inverse Fast Fourier Transform (IFFT) unit 112 modulates the frequency domain data supplied by the data symbol encoder 106 and produces time domain signals to be transmitted. The time domain signals are then supplied to a digital-to-analog converter (DAC) 114 where the digital signals are converted to analog signals. Thereafter, the analog signals are transmitted over a channel to one or more remote receivers.
FIG. 1B is a block diagram of a remote receiver 150 for a conventional multicarrier modulation transmission system. The remote receiver 150 receives analog signals that have been transmitted over a channel by a transmitter. The received analog signals are supplied to an analog-to-digital converter (ADC) 152. The ADC 152 converts the received analog signals to digital signals. The digital signals are then supplied to a Fast Fourier Transform (FFT) unit 154 that demodulates the digital signals while converting the digital signals from a time domain to a frequency domain. The frequency domain digital signals are then supplied to a frequency domain equalizer (FEQ) unit 156. The FEQ unit 156 performs an equalization on the digital signals so the attenuation and phase are equalized over the various frequency tones. Then, a data symbol decoder 158 receives the equalized digital signals. The data symbol decoder 158 operates to decode the equalized digital signals to recover the data, or bits of data, transmitted on each of the carriers (frequency tones). In decoding the equalized digital signals, the data symbol decoder 158 needs access to the bit allocation information and the energy allocation information that were used to transmit the data. Hence, the data symbol decoder 158 is coupled to a received bit allocation table 162 and a received energy allocation table 160 which respectively store the bit allocation information and the energy allocation information that were used to transmit the data. The data obtained from each of the frequency tones is then forwarded to the forward error correction (FEC) unit 164. The FEC unit 164 performs error correction of the data to produce corrected data. The corrected data is then stored in a buffer 166. Thereafter, the data may be retrieved from the buffer 166 and further processed by the receiver 150. Alternatively, the received energy allocation table 160 could be supplied to and utilized by the FEQ unit 164. The received energy allocation table 160 could also be incorporated into the FEQ unit 164.
The bit allocation tables and the energy allocation tables utilized in the conventional transmitter 100 can be implemented as a single table or as individual tables. Likewise, the bit allocation tables and the energy allocation tables utilized in the remote receiver 150 can be implemented as a single table or as individual tables. Also, the transmitter 100 is normally controlled by a controller, and the remote receiver 150 is normally controlled by a controller. Typically, the controllers are programmable controllers.
The transmitter 100 and the remote receiver 150 illustrated in FIGS. 1A and 1B, respectively, optionally include other components. For example, the transmitter 100 could add a cyclic prefix to symbols after the IFFT unit 112, and the remote receiver 150 can then remove the cyclic prefix before the FFT unit 154. Also, the remote receiver 150 can provide a time domain equalizer (TEQ) unit between the ADC 152 and the FFT unit 154.
A data transmission system normally includes a central office and a plurality of remote units. Each remote unit communicates with the central office over a data link (i.e., channel) that is established between the central office and the particular remote unit. To establish such a data link, initialization processing is performed to initialize communications between the central office and each of the remote units. For purposes of the discussion to follow, a central office includes a central modem and a remote unit includes a remote modem. These modems are transceivers that facilitate data transmission between the central office and the remote unit. In general, the initialization processing for a multicarrier modulation transmission system includes the general operations of activation and acknowledgment of initialization requests, transceiver training at both the central office and the remote unit, exchange of rate requests between the central office and the remote unit, channel analysis, and exchange of transmitter settings (i.e., bit allocation tables) between the central office and the remote unit.
With respect to a multicarrier modulation transmission system in which a plurality of carriers (e.g., frequency tones) are used to transmit data between the central office and the remote unit, namely an ADSL system, the initialization processing conventionally carried out is known in the art. The T1E1 ADSL Standard describes an initialization process that transceivers must adhere to comply with this standard. The T1E1 ADSL Standard is hereby incorporated by reference.
FIGS. 1C and 1D are flow diagrams of conventional initialization processing 170. The initialization processing 170 is used to initialize communication links between a central office (CO) and a remote unit (RU). The initialization processing 170 is exemplary of the initialization processing described in the T1E1 ADSL Standard.
The conventional initialization processing 170 initially begins with activation and acknowledgment processing 172 at both the central office (CO) and the remote unit (RU). Next, transceiver training 174 is performed at both the central office (CO) and the remote unit (RU).
The central office (CO) then forwards 176 downstream rate requests and message information to the remote unit (RU). Next, the remote unit (RU) forwards 178 upstream rate requests and message information to the central office (CO). Often, the upstream rate requests are actually forwarded with the forwarding 176 of the downstream rate requests, and then mirrored back by the forwarding 178.
The rate requests are initially provided by a network, such as a telephone network, that typically couples to the central office (CO). The rate requests are usually provided before or just after the activation and acknowledgment processing 172. Also, the message information may, for example, identify features, options or limitations associated with the associated transceiver.
After exchanging the rate requests, the initialization processing 170 operates to determine 180 channel and noise characteristics at both the central office (CO) and the remote unit (RU). The channel and noise characteristics can be determined by transmitting a predetermined sequence (in both directions) over a link between the central office (CO) and the remote unit (RU). Then, signal-to-noise ratio (SNR) is determined tone by tone based on the channel and noise characteristics. Thereafter, upstream bit and energy allocation can be determined 182 at the central office (CO), and downstream bit and energy allocation can be determined 184 at the remote unit (RU). The bit and energy allocation operates to assign a number of bits to each of the carriers (e.g., frequency tones) of the multicarrier modulation transmission system. Likewise, the energy allocations assign energy levels to each of the carriers of the multicarrier modulation transmission system. In obtaining these allocations, the central office (CO) chooses an upstream rate which is often one of the available requested upstream rates it is able to satisfy, and the remote unit (RU) chooses a downstream rate which is often one of the available requested downstream rates it is able to satisfy.
Next, the remote unit (RU) forwards 186 the chosen downstream rate, a downstream performance margin for the chosen downstream rate, and message information all to the central office (CO). The message information may, for example, include estimated loop attenuation or total bits per symbol supported. A decision block 188 then determines whether the chosen downstream rate is less than all of the downstream requested rates. If the chosen downstream rate is less than all of the requested downstream rates, the data link between the transceivers is unable to support the requested level of service, and thus, the initialization processing 170 must be restarted.
On the other hand, if the chosen downstream rate is not less than all of the requested downstream rates, then the initialization processing 100 continues. In particular, the central office (CO) renders 190 a final decision on the downstream rate.
In many cases, the central office (CO) follows the downstream rate chosen by the remote unit (RU). After making the final decision on the downstream rate at the central office (CO), the central office (CO) forwards 192 upstream performance margin, selected rate for the upstream and downstream transmissions, and message information to the remote unit (RU). The message information may, for example, include estimated loop attenuation or total bits per symbol supported. Next, a decision block 194 determines whether the selected upstream rate is less than all of the requested upstream rates. If the selected upstream rate is determined to be less than all of the requested upstream rates, the initialization processing 170 must be restarted.
On the other hand, when the selected upstream rate is not less than all of the requested upstream rates, the initialization processing 170 continues. Specifically, the central office (CO) sends 196 an upstream bit and energy table to the remote unit (RU). In addition, the remote unit (RU) sends 198 a downstream bit and energy table to the central office (CO). In the case where the central office (CO) overruled the initially chosen downstream rate with the final downstream rate, the remote unit (RU) must re-determine the downstream bit and energy allocations before sending the downstream bit and energy table to the central office (CO). Following block 128, the initialization processing 170 is complete and ends.
There are several drawbacks to the conventional initialization processing. One drawback is that at two different points in the initialization processing the central office and the remote unit are required to exchange data. Each data exchange adds delay to obtaining initialization and thus the establishment of a data link over which data can be transmitted. Another drawback is that the central office only has partial information about the extent of data that downstream transmissions can suitably handle. As a result, multiple iterations through the initialization processing are often required. Each additional iteration also adds to the delay for obtaining initialization and the establishment of a data link. The conventional initialization processing also has to be restarted if the data link cannot provide any of the requested rates for upstream or downstream data transfer. Further, even when the data link is established, the data link will sometimes fail and then require re-initialization. These drawbacks of the conventional initialization processing induce inefficiencies and inflexibility into a data transmission system and thus hinder performance.
Thus, there is a need for improved initialization techniques for initializing data transfer between a pair of transceivers in a multicarrier modulation transmission system.